1. Field of the Invention
The invention relates to methods and apparatus for precise time and/or location tagging of an event, and particularly for determining time and location of an event with DGPS accuracy using a high-latency communication channel.
2. The Prior Art
FIG. 1 illustrates a conventional global positioning system (GPS) arrangement. A GPS receiver 100 receives encoded signals from satellites S1, S2, S3, and S4. Receiver 100 includes a clock which determines a pseudo-range to each satellite by comparing a locally-generated code with a code contained in the satellite signal to determine the arrival time of the signal. Receiver 100 records pseudo-ranges R1, R2, R3, and R4 from their respective satellites. Inaccuracy of the clock in receiver 100 used to time the signal arrivals results in an unknown error in the pseudo-range measurements. Receiver 100 computes from the four pseudo-range measurements its longitude, latitude and altitude from the earth's center and its receiver clock error. The computed clock error is used to synchronize the receiver clock with the synchronized atomic clocks of the satellites. The computation gives receiver position to within about 15 meters and time to within 100 ns. Sources of measurement error include satellite clock errors, satellite orbital ephemeris errors, atmospheric delays in the ionosphere and troposphere, receiver noise, and multipath (signal reflections). With these errors, accuracies on the order of 15 meters are achievable. With "Selective Availability" (SA) in effect, an intentional error of greater magnitude is introduced by the U.S. Department of Defense (DoD) so the position solutions computed by a stationary receiver wander within a circle of 100 meter radius. Better accuracy is required for many applications. Differential GPS (DGPS) can be used to counteract most of these errors. Accuracies on the order of few meters are possible with DGPS, some units even achieving accuracies to 1 centimeter.
FIG. 2 illustrates a typical local-area DGPS installation. A reference DGPS receiver unit 200 is accurately placed at a known location 210. Receiver unit 200 acts as a static reference, computing and transmitting error corrections to other GPS receivers in the area. For example, an error message is transmitted as shown at 220 to a roving GPS receiver 230 at a location 240. The error message can take a variety of forms. The main technique is for the error message to be sent by telemetry. Roving receivers (such as receiver 230) process the error correction message and the received satellite signals to compute a position solution of enhanced accuracy In computing the error corrections, the reference receiver uses its known position and the known locations of the satellites to compute a theoretical arrival time. The difference between the computed theoretical time and the actual transmission time is the error (or delay).
The roving receivers continually receive a complete list of errors for all satellites and apply the corrections for the particular satellites they are using. The error correction messages are typically transmitted by a one-way radio link. Each of the roving GPS units is equipped with a radio receiver to receive and decode the corrections for use in computing a corrected position fix. See, for example, E. BLACKWELL, Overview of Differential GPS Methods, NAVIGATION, Vol. 32, No. 2, 1985, pp. 114-125.
FIG. 3 shows a prior art local-area DGPS arrangement. Reference unit 200 comprises a GPS receiver 300, a processor 310, and a radio transmitter 320. Receiver 300 computes a position solution X.sub.0, Y.sub.0, Z.sub.0 from received satellite signals. The location of antenna 305 is accurately surveyed and known to be at location X.sub.0, Y.sub.0, Z.sub.0. Processor 310 uses the computed position solution from receiver 300 and the known location data to compute an error signal which is broadcast by radio transmitter 320 to all rover units in the area. Each rover unit 230 has a radio receiver 330 which supplies the error signal to processor 340. GPS receiver 350 computes a position solution from the received satellite signals, and processor 340 computes a corrected solution. The reference unit of a local-area DGPS system develops a scalar correction for each satellite pseudorange, which means that it does not try to break the observed error into components. The validity of the correction is reduced with increasing distance between the reference unit and a rover unit (spatial decorrelation). A local-area DGPS system thus serves a relatively small area of 500-800 km radius. Also known are wide-area DGPS systems in which a network of reference stations is used to develop vector corrections for each satellite in view. These systems estimate separately the error components due to satellite ephemeris, satellite clock and atmospheric delay. Wide-area DGPS is effective because the error components have differing decorrelation distances and times. A wide-area DGPS data stream can serve an entire continent. See U.S. Pat. No. 5,155,490 to Spradley, Jr. et al. for an example of a GPS system using a network of fixed stations to compute error-correction data. See also P. GALYEAN, The Acc-Q-Point DGPS System, PROC. ION GPS-93, Salt Lake City, 22-24 Sep. 93, pp. 1273-1283.
With either type of DGPS, timeliness of error-correction data is critical. As shown in FIG. 4, position solutions over time of a stationary GPS receiver with SA in effect wander within a radius of about 100 m due to the SA dynamic of 1 cm/sec with an acceleration of 1 cm/sec.sup.2 (approximately 1 mG). Some time is required to compute the error-correction data, and additional time is needed to transmit the error-correction message. The sum of these times is called the "latency." FIG. 5 shows the effect of error-correction data latency. Dashed lined 500 shows the net error with uncorrected GPS. Curve 510 (not to scale) shows the net error for a "corrected" DGPS position fix. The error of the DGPS fix is substantially less than an uncorrected fix if the correction data is less than 15-20 seconds old, is still beneficial up to about 30-40 seconds and has the effect of increasing net error if more than 40 seconds old.
A radio link is adequate for transmitting DGPS error signals in many applications, such as in a harbor for use by ships. Some links operate at transmission rates low enough to require more than 10 seconds to transmit the corrections for all the visible satellites. Experience shows that an update rate of once every five seconds is much better, especially with SA in effect.
Some jobs do not require "real time" corrections, such as surveying or cartography. In these situations, the roving receiver records each measured position and the exact time it made the measurement. This data is later merged with the corrections recorded at the reference receiver for a final "clean-up" of the data. This is sometimes called "post-processed" DGPS.
Another variation is "inverted" DGPS, which can be useful in fleet management. Each vehicle of a fleet reports its position periodically to a base station where the position fixes are corrected. The position fix at each vehicle is only known to "raw" GPS accuracy, but the base (reference) station can pinpoint the location of each vehicle with DGPS accuracy. An example of an inverted DGPS system is described in U.S. Pat. No. 5,223,844, in which a cellular telephone link is used to transmit uncorrected GPS. See also International Patent Publication No. WO-89/12835, published 28 Dec. 1989, which discloses a vehicle locating system having a navigation unit, microcomputer, and cellular telephone mounted on the vehicle and interconnected so that a central station can monitor the location of the vehicle by receiving position information from the navigation unit via the cellular telephone link.
To summarize, a roving receiver can compute its location to within 100 meters with SA in effect or to within about 30 meters without SA. A rover unit which can obtain error correction signals with a latency less than about 30-40 seconds can compute its position with greater accuracy than uncorrected GPS, though a latency of less than about 20 seconds is preferred for best accuracy. Post-processed GPS provides optimum accuracy, but the corrected fixes are not available until raw fixes are merged (later, and back at a home base) with the correction data; the rover knows its position only to "raw" GPS accuracy. Inverted DGPS likewise has the disadvantage that the rover can compute its position only to "raw" GPS accuracy because it does not have the error correction data.
Those systems which allow the roving unit to differentially correct its GPS position fixes, such as harbor navigation systems for ships, depend on the availability of a communication link having minimum latency to transmit the error correction data to the roving unit. Such communication links are unavailable in many locations, such as outside the operating range of cellular telephone transmitters and away from harbors or other populated areas having DGPS transmitter beacons.
Some users require DGPS accuracy in remote locations accessible only by satellite relay link. Examples include placement of wireless network repeaters, asset tagging, and precise mapping of the fringe-reception areas of cellular networks, time and location tagging of instrument readings, etc. It is therefore desired to provide a system in which a roving GPS receiver can obtain and make effective use of error correction data from a reference station via a communication link having a significant delay, e.g., a delay which renders the error correction data so stale as to be otherwise unusable.